Fabric wipers used in cleanroom applications require low particle generation when tested in air and water environments. In addition, cleanroom wipers must exhibit adequate absorbency rates and capacities. Unfortunately, particle generation and absorbency properties for many fabrics are many times mutually exclusive of each other. For example, untreated 100% polyester fabrics generate very low wet and dry particle counts but provide almost no absorbency. On the other hand, cotton fabrics or fabrics containing woodpulp exhibit high absorbency rates and capacity but typically generate unacceptably high wet and dry particle counts.
In the past, commercially available non-patterned spunlaced woodpulp/polyester fabrics (55 wt. % woodpulp/45 wt. % polyester) have proved adequate when used in Class 100 cleanroom environments. Federal Standard 209E, Sept. 11, 1992, defines airborne particulate cleanliness classes of air in cleanrooms using both English and metric units, and specifies that Class 100 air shall have no more than 100 particles (0.5 micrometer or larger) per cubic foot, or the equivalent metric designation of no more than 3530 particles (0.5 micrometer or larger) per cubic meter for Class M 3.5 air. Although Class 100 environments may be currently acceptable for non-sensitive operations, it has become increasingly desirable to have even lower particle counts for sensitive high-tech cleanroom applications.
U.S. Pat. No. 3,485,706 (Evans) discloses hydroentangling fibrous webs to produce textile-like patterned nonwoven fabrics. The hydroentanglement process calls for imparting high energy water jets (i.e., usually between about 200 and 2,000 psi) to a fibrous web to entangle the web and produce a spunlaced fabric. In FIG. 40 of Evans, a continuous commercial process is depicted wherein the fabric is subsequently dewatered by one or more squeeze rollers. Unfortunately, the application of high impact energy and squeeze roll dewatering produces fabrics which are typically unacceptable for sensitive high-tech cleanroom wiper applications.
Numerous examples in Evans disclose patterned spunlaced fabrics. Typically, the patterned spunlaced fabrics are fabricated of 100% synthetic textile staple fibers (e.g., polyester). Patterning takes place during hydroentanglement treatment by supporting the fibers on an apertured patterning member and then passing the fibers through a series of water jet banks. In addition, there are a few samples disclosed in Evans which demonstrate the use of relatively short cellulosic fibers in combination with synthetic staple fibers. These samples were made on table washers where the belt speed was very slow. Moreover, some of these samples were formed using predominantly cellulosic fibers (i.e., less than 50 wt. % synthetic fibers).
However, when a continuous commercial process was considered for making patterned spunlaced fabrics formed of synthetic fibers and woodpulp and/or woodpulp-like fibers, the conventional wisdom was that, although webs of 100% synthetic fibers could be successfully hydroentangled and patterned at commercial speeds, webs containing woodpulp and/or woodpulp-like fibers could not be formed on an apertured patterning member without destroying web integrity and/or generating large amounts of wet and dry particles. The wisdom was that supporting a synthetic/woodpulp web on an apertured patterning member would cause the woodpulp fibers to be washed out of the web through the openings in the patterning member when the web was treated with high energy water jets during hydroentanglement. In addition, it was believed that low process speeds (i.e., speeds below about 135 yds/min) would be required in order to overcome the problem of fabric wrinkling caused by excessive water carryover. Therefore, synthetic/woodpulp spunlaced fabrics were not patterned by high speed commercial hydroentanglement processes for fear that the web would lose its integrity during hydroentanglement treatment and/or that the fabric would exhibit numerous post-treatment wrinkles due to water carryover.
Due to the problems inherent in the prior art, the applicant recognized the need for a spunlaced fabric which provides an adequate degree of absorbency yet very low wet and dry particle counts. In this regard, the applicant has surprisingly found that patterned spunlaced fabrics formed of synthetic fibers and woodpulp and/or woodpulp-like fibers provide low wet and dry particle counts yet good absorbency when processed under certain conditions. These conditions allow such patterned spunlaced fabrics to be made at commercial speeds without woodpulp washout or fabric wrinkling. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.